Structure of inner sleeve of electronic gripper is shown in Figure 1. Plastic part is a double-section cylindrical shell. Height of upper section is 12 mm and outer diameter is φ16 mm. Height of lower section is 45 mm and outer diameter is φ30 mm. There is an outward protrusion of 1 mm on one side in the middle part of pitch cylinder, which is φ32 mm. Outer wall of upper cylinder is provided with front and rear clamping slots. These two slots can only be pulled sideways to remove core. Outer wall of lower cylinder is equipped with an elliptical open groove. Features in groove include 1 boss, 2 side undercuts, 1 pad, 1 boss and 1 sink hole. Inner wall of lower cylinder is provided with 2 rectangular undercuts. Plastic part material is ABS, and shrinkage rate is 1.052~1.058.

 

Figure 1 Inner sleeve of electronic gripper

Structural characteristics of plastic part determine mold structure. The first consideration is demoulding mechanism of plastic part, followed by pouring method, and finally cooling system design. Combined with structure and characteristics of plastic part, the following steps should be considered for demoulding.

(1). Demold two rectangular undercuts on inner wall of lower cylinder of plastic part. If an inclined push rod is used for molding and side core pulling and demoulding, its head is prone to interference due to space constraints when pushing out, so it is advisable to use an inward slider for core pulling and demoulding. For two undercuts, use front and rear adduction sliders to perform core pulling to demold, as shown in Figure 2(a). Distance between the two is adduction distance d, and core pulling distance of one-sided adduction slider is s. Design of demoulding mechanism: First, outer wall molding part of plastic part is separated from inner wall molding part by following parting surface. Inner wall molding part is core, then two inverted molding parts are divided on core, namely front and rear adduction sliders; when demoulding, need to move front and rear adduction sliders by F3 and F4 respectively by s distance, as shown in Figure 2(b).

 

Figure 2 Inner wall undercut and demoulding
1. Rear adduction slider 2. Plastic part 3. Front adduction slider 4. Front wear-resistant plate 5. Core 6. Rear wear-resistant plate 7. Right Huff slider 8. Front bending pin 9. Spring 10 .Back bend pin
The two slide blocks are installed on core through their respective wear-resistant plates. Wear-resistant plates also play a role in limiting core pulling of adducting slide block. Core pulling distance s of one-sided adducting slide block is 3.5 mm. Core-pulling driving force and reset driving force of front and rear adduction slide blocks have different power sources. As shown in Figure 2(b), driving force of front adduction slide block comes from spring, reset driving force comes from bent pin on the right Huff slider. When right Huff slider stays in mold closed position, forward bending pin installed at its lower end drives front adduction slider to move to the left, forcing spring to be compressed, so that spring maintains an extension tendency. Likewise, when mold is closed, rear bending pin will force rear adduction slider to stay in undercut forming position. When mold is opened at lower parting surface, right Huff slider moves in F0 direction. F0 direction can be decomposed into F1 and F2 directions. Forward bending pin also moves in these two directions. Movement of forward bending pin in F1 direction will force front adduction slider to retract and pull in F3 direction under drive of spring, thereby achieving separation of forming part of front adduction slider from corresponding undercut feature; similarly, rear bending pin will also force rear adduction slider to move in F4 direction, completing separation of forming part of rear adduction slider from corresponding undercut feature, realizing demoulding of two undercuts on inner wall of plastic part.

(2). Demolding of outer wall of plastic part. As shown in Figure 3, since middle of outer surface of lower cylinder of plastic part is drum-shaped, Haugh sliders should be installed on outer surface of lower cylinder for demolding. Molded parts are left and right Huff sliders. Since front and rear slots on both sides of upper cylinder can only be demoulded by side core pulling, molded parts on outer surface of upper cylinder are integrated on left and right Huff sliders. Considering features of bosses, pads, bosses and countersunk holes in oval open groove on outer wall of lower cylinder, side core pulling can only be used to demold. Molding parts of these features are integrated on left Huff slider.

 

Figure 3 Side oblique push ejection mechanism
1. Front adduction slider 2. Left Huff slider 3. First oblique push rod 4. Second oblique push rod 5. Rear adduction slider 6. Right Huff slider 7. Spring 8. Stopper rod 9 . Incline push seat 10. Incline push seat spring 11. Plastic part 12. Upper pressure block 13. Upper panel part 14. Center insert
Since the two undercut moldings inside oval open groove be core-pulled synchronously with left Huff slider, it is necessary to set up a double oblique pushing and adduction-first core-pulling mechanism in left Huff slider to complete demoulding of two undercuts, so as to ensure that left Huff slider can achieve side core pulling and demoulding on one side of outer wall of plastic part. As shown in Figure 3, two undercuts are formed using the first and second oblique push rods. The first and second oblique push rods are installed in two oblique push grooves opened on left Huff slider. After the two complete side core pulling along F3 and F4 directions respectively, they then follow Huff slider to move laterally along F6 direction. Implementation steps of this action in mold: When mold is opened, upper parting surface P1 is opened first, left and right Huff sliders are separated at the middle parting surface P3. After P3 surface is opened and outer wall of plastic part is completely demoulded, lower parting surface P2 is opened again.
Movement of side oblique push ejection mechanism: When left Half slider and upper insert plate are separated at upper parting surface P2, left Half slider moves in F5 direction, movement in F5 direction can be decomposed into F6 direction (horizontal direction ) and F7 direction (vertical direction) movement. Left Huff slider does not move relative to plastic part in vertical direction, but will move relative to it in horizontal direction. When left Huff slider moves in F6 direction, since blocking rod is installed in upper panel, push rod is fixed. Push rod will resist inclined push seat in F6 direction, so that it will not move in F6 direction, but inclined push seat will move downward relative to push rod in F7 direction. Since the first inclined push rod is installed in the first inclined push groove opened on left Huff slider, and its tail end is installed on inclined push seat, the first inclined push rod does not move in F6 direction, but is driven by the first inclined push groove of left Huff slider to pull core in F3 direction to complete ejection action of plastic part being reversed. When the first inclined push rod completes side core pulling, inclined push seat has moved H distance relative to blocking rod, and blocking rod loses its restriction on inclined pushing seat in F6 direction. Driven by stretching of inclined pushing seat spring, inclined push seat moves the first inclined push rod in F6 direction, so that the first inclined push rod is reset to position before upper parting surface P2 is opened. Then tilt push seat and the first tilt push rod follow left Huff slider and move appropriate core pulling distance in F6 direction, waiting for mold to close and reset. When inclined push seat is installed in left Huff slider, its upper end is provided with an upper pressure block to limit position, and upper pressure block is provided with an avoidance groove for blocking rod. Action principle of second inclined push rod is same as that of the first inclined push rod.

Mold has a two-plate structure, as shown in Figure 4, and is opened three times. The first parting is when upper parting surface P1 opens, left and right Huff sliders slide down, and middle parting surface P3 opens. Opening of P1 surface realizes core pulling and demoulding of inner wall of upper cylindrical section of plastic part. Opening of P3 surface realizes core pulling and demoulding of outer wall of plastic part and the two side undercuts in oval open groove. At the same time, front core pulling slider is retracted to complete side core pulling and demoulding of an undercut on inner wall. After movement of the two Huff sliders is completed, lower parting surface P2 is opened, and back bending pin is driven to retract core-pulling slider to complete side core-pulling demoulding of remaining undercut of inner wall of plastic part. Mold has  2-cavity layout. Single-cavity pouring system uses a side gate for pouring, and runners are set on the left and right Huff sliders. Cooling ducts must be installed on left and right Huff sliders and cores, and diameter of cooling ducts is φ8 mm. Final demoulding of plastic parts is completed by pushing multiple push rods with push plate.

 

Figure 4 Mold structure